1. Field Of The Invention
The present invention relates to a process for the production of an oriented, heat-set polyester film containing an admixture of polydiorganosiloxane in a quantity of from 0.001 to 2% by weight and optionally further additives comprising inert inorganic and/or organic particles. The process comprises the steps of melting the thermoplastic in an extruder, extruding it through an appropriate die to yield a cast film, then quenching, reheating, stretch orienting, and heat-setting the film.
2. Prior Art
Because of their superior qualities such as their tensile strength, their tear resistance, their modulus of elasticity, their transparency, their chemical and thermal stability, etc., biaxially or multiaxially oriented films made of polyester, particularly polyethylene terephthalate, find great application in many technical areas, such as the reprographic sector, as a dielectric for capacitors, as base films for magnetic recording media, such as audio-, video-, and computer tapes, for magnetic cards, as well as floppy discs.
The polyester films must fulfill specific requirements for the various areas of usage, which must be adjusted during the film production with the aid of raw material formulations or by means of a specific process technology. Thus, for example, polyester films, which are processed into magnetic tapes, should display a lower friction coefficient, as well as high scratch and abrasion resistance during continuous stress, so that the magneto-electric qualities are not adversely influenced. Beyond that, the films that are provided with these qualities should, from a commercial standpoint, be economical to produce. For example, any thermoplastic scrap obtained during the film production should, as much as possible, be completely recyclable as reclaim to the film production process, without there being any negative influence on the quality of the film which was partially produced from reclaimed material.
A film containing polydiorganosiloxane in the area of 0.01 to 2% by weight, is already known from Japanese Pat. No. 50-74,650. With the known film, however, the polysiloxane is added during the polycondensation reaction in the production process of polyester. This may be viewed as a disadvantage, since it is unavoidable that parts of the polydiorganosiloxane decompose under the reaction conditions of polycondensation, whereby the cleavage products once again result in a detrimental effect on the qualities of the films produced from the respective polyesters.